Typealiases should have their own separate pages

[IgnatBeresnev]

At the moment, typealiases don't have their own pages and can only be found among the package's types, which is not always convenient and is difficult to link to.

Typealiases should have dedicated pages, with persistent index.html generated for them, similar to other declarations.

Things to consider/keep in mind:

1. Previously generated anchor links should be preserved if possible (so compatibility with previous versions of Dokka, especially when linking to external documentation).
2. If not already, the description of the typealias declaration on the package page should be changed to "brief", similar to other elements.
3. The dedicated page for the typealias should display full documentation (all KDoc paragraphs, tags, etc).
4. Check that the search and similar common features work (for instance, see Kotlin's typealias is not handled correctly #2311)
5. Some internal refactoring may be needed as DTypeAlias is not handled the same way as DClasslike or any other documentable, so it needs to be checked/supported additionally.

[IgnatBeresnev]

Related: #2311, #1202

[IgnatBeresnev]

Can probably be closed after this is implemented: #1667

[ilya-g]

Related: #2971

[vmishenev]

This change requires some refactoring (documentable or/and page model) that should be discussed.
Currently, we have ClasslikePage (see Pages.kt) that can contain actual type alias.
Also, we have mergeImplicitExpectActualDeclarations so one page can have a classlike and a non-actual typealias with the same name.
Options:

• Rename ClasslikePage to ClasslikeOrTypeAliasPage
• Introduce an interface DType (DTypeAlias: DType, DClasslike: DType) and rename ClasslikePage to DType
• Introduce TypeAliasPage (only if we have only typealiases, not classlikes) and use ClasslikePage if we have a classlike with the same name
  ...

[qwwdfsad]

I wonder why ClasslikePage does not imply it could refer to typealias though, it looks reasonable as it is right now

[vmishenev]

I wonder why ClasslikePage does not imply it could refer to typealias though, it looks reasonable as it is right now

Yes, but in documentable mode we have independent DTypeAlias and DClasslike (that even has a scope).

[vmishenev]

This is implemented since Dokka 1.9.0